I personally use a RAID enclosure from Raidon with two drives in a mirrored configuration. Something like the GR3630-2S-SB2:
http://www.newegg.com/Product/Product.aspx?Item=N82E16816142002
(I've used Accordance RAID systems in the past, before my current Raidon).
Then I buy an additional HD tray and rotate the 3 drives occasionally to an offsite backup. Basically the cost is about $345 total, $75*3 drives+$120 for the unit, and I get 3 copies of the data. Additional offsite backups run $105 ($75 drive+$30 tray).
I've found this the easiest to understand (for end users) and a very cheap and reliable storage method. I've had the nightmare of backing up to 8 different tapes, only to go to restore and find out that 6 tapes are empty, 1 is bad, and 1 has data from 6 months back. Because you're actually running your OS off of the drives, you don't get into that situation. The main key is to make sure that you pull drives when the system is off (so that all files are written to disk), and that you insert drives when the system is ON (so that the raid array knows the drive is replaced and doesn't corrupt your data).
NEVER INSERT A DRIVE WHILE THE SYSTEM IS OFF.
I have files from over 15 years ago on my main desktop computer, and the only files I'm missing are from my original 286 before I started this backup method.
This solution can be retrofitted on any desktop computer. All you need to do is install an eSata card (if you don't have an eSata port) and then purchase additional drives (SAME model number OR larger capacity as the current drive in the computer). Simply stick the current HD into the raid array FIRST, then put the new empty drives in SECOND, and it'll automatically mirror. Set the computer to boot from eSata and you're all set.
If you ever want to upgrade capacity, simply buy 3 larger drives and it will automatically mirror those drives to a larger capacity for you.